1909-10.] Observations on some Spark-Grap Phenomena. 227 
shown in fig. ix., and fig. x. gives an example of a hole made just beyond 
the + (spherical) electrode, in which case it will be noted that two distinct 
paths are available. 
V. An attempt was made to observe the influence of low pressures in 
F iq. Wii\ 
air and other gases on these phenomena, and on the Lullin effect. A strong 
glass bell-jar was accordingly made to contain the spark-gap and a con- 
trivance for supporting or suspending the dielectric. The dimensions of 
the jar were approximately : — height, 40 cm. ; average thickness, 1 cm. ; 
fKj »* • 
mean inside diameter, 25 cm. It was made in two parts for convenience of 
access to the interior, and was fitted with stop-cocks for exhaustion and 
for admission of other gases. In the first experiment the air was exhausted 
to a pressure of about 4 mm. Hg. and a spark passed. Instantly the lower 
half of the jar split, with a loud report, from top to bottom — one great open 
